steel So sorry to hear what has happened. 
I don't know how long you were with him, but reading over the last few posts, I can now see the advantage of not getting emotionally involved too quickly ever
You say that if you'd been considering ending it with him/someone because there was potentially someone else on the horizon, then you'd be behaving differently. You might, but that is you. A lot of people wouldn't.
I don't think you're a bad judge of character. I suspect that everything he said/showed he felt was true, but there was this other thing that had snuck in over the past month and was running parallel.
He could well have been wrangling with an emotional maelstrom himself and feeling very confused: really likes you, but then this blast from the past arrived and threw a massive spanner in the works. He was probably just giving himself the best shot of making the right decision for himself. After all, as much as we all consider other people's feelings, ultimately, we have to do what is best for ourselves.
I know that's shit for you but, being pragmatic about it all, I don't think it sounds like he was being a bastard. He probably feels like a coward for making a decision that has hurt someone he cares about. (Afterall, you're thinking/saying all sorts of horrible things about yourself because of what has happened). And he has been honest with you now.
And too be honest, cooling off and fizzling out isn't any better. That leads people to post frantically on here wondering what's happened, trying to second guess someone else's feelings/motives/intentions... It causes people to obsess and question themselves over an extended period of time. I think what he has done is more honourable than that. (Note more honourable, not honourable)
Unless you really don't like someone, ending a relationship is draining - there are 'what ifs..?' in both directions. It's often hard to know if it was the right decision or not until it's very much after the fact and far too late to be doing anything about it.